An active matrix substrate used in liquid crystal display devices and the like has a switching element such as a thin film transistor (hereinafter, “TFT”) in each pixel. An active matrix substrate including TFTs as switching elements is referred to as a TFT substrate.
TFTs having an amorphous silicon film as the active layer (hereinafter “amorphous silicon TFTs”) or TFTs having a polycrystalline film as the active layer (hereinafter “polycrystalline silicon TFTs”) are widely known.
The use of an oxide semiconductor as the material of the TFT active layer, instead of amorphous silicon or polycrystalline silicon, has been recently proposed. Such a TFT is referred to as an “oxide semiconductor TFT.” Oxide semiconductors have a higher mobility than amorphous silicon. Therefore, the oxide semiconductor TFT can operate at a faster speed than the amorphous silicon TFT. Oxide semiconductor films are made by a simpler process than the polycrystalline silicon film.
Patent Document 1 discloses a method of manufacturing a semiconductor device by which miniaturization of oxide semiconductor TFTs is accomplished while maintaining excellent electrical characteristics.
Meanwhile, in recent years, liquid crystal display devices and the like become higher resolution, a decrease in aperture ratio of the pixels has become a problem. The aperture ratio of the pixel refers to the proportion of area taken up by the pixel in a display region (in a transmissive liquid crystal display device, for example, this refers to an area through which light passes through to contribute to display), and below, this is simply referred to as the “aperture ratio.”
In particular mid- to small-size transmissive liquid crystal display device for mobile devices have a small display region, and thus, the individual pixels are also small in area, which means that there is a marked decrease in aperture ratio due to increased resolution. Furthermore, if the aperture ratio of liquid crystal display devices used for mobile devices is decreased, then in order to attain a desired brightness, the luminance of the backlight needs to be increased, which can increase the amount of power consumed.
In order to attain a high aperture ratio, the area taken up by elements made of non-transparent materials such as TFTs and auxiliary capacitance provided for each pixel can be made small, but there is a minimum required size for the TFTs and auxiliary capacitances in order for them to perform their functions. If oxide semiconductor TFTs are used as the TFTs, there is an advantage that the TFTs can be made smaller than if amorphous silicon TFTs are used. In order to maintain the voltage applied to the liquid crystal layer of the pixels (referred to as “liquid crystal capacitance” in electrical terms), the auxiliary capacitance is provided as a capacitance provided to be electrically parallel to the liquid crystal capacitance, and in generally, at least some of the auxiliary capacitance overlaps the pixels.